Aqueous colloidal silicas, such as Ludox.RTM. SM and HS aqueous colloidal silicas sold by Du Pont, are widely used as binders to form molds for use in the investment casting industry.
Ceramic shell investment casting is a process that simplifies the manufacture of complex metal parts. Detailed metal parts may be cast in an almost unlimited selection of metals and alloys, to precision tolerances with fine surface finish.
A typical investment casting process comprises encasing an expendable wax or plastic pattern of the piece to be produced in a ceramic shell by dipping the pattern in a slurry of refractory grain and silica binder, coating the wet slurry on the pattern with a dry refractory grain, or stucco, and then drying the pattern. These steps are repeated until a necessary thickness of refractory is built up around the pattern to provide the required shell strength. The expendable pattern is melted out, the shell is fired to eliminate pattern residues, and metal is poured into the hollow shell. After solidification of the metal, the shell is broken away and discarded.
The pattern is prepared in a die, and must be washed to remove mold release agents, such as silicone, and greases. This wash is necessary to assure good wetting of the pattern during a prime coat dipping step because colloidal silica slurries do not "wet", or adhere to, the surfaces of a wax or plastic pattern, but rather bead and run off the pattern surfaces. Alcohols, such as ethanol, ketones, such as methyl ethyl ketone, and fluorocarbons, such as Freon.RTM. TF are often used as pattern washes.
Wetting agents and latex additives are sometimes added to the first or "prime" coats of slurry to improve adhesion to the wax.
Coating slurries are prepared by mixing a colloidal silica and a refractory grain. Typically two slurries are prepared: a prime coat slurry and a back-up slurry. The prime coat slurry typically contains a surfactant to promote uniform wetting of the pattern. Surfactant levels are usually from 0.1 to 1.0% based on the binder content of the slurry.
The prime coat slurry usually has a high refractory grain content and viscosity (15-35 seconds --#5 Zahn cup) and normally is used to apply the first two coats to the pattern. A fine grain, 200 to 300 mesh, is used in the prime coat slurries because the fine grain forms a smooth surface, closely reproducing the surface of the wax pattern.
Subsequent coats are applied to the pattern using the back up slurry. The back up slurry usually has a somewhat lower grain content and viscosity (7-20 seconds --#5 Zahn cup) than the prime coat slurry, and uses larger mesh grains.
The exact pattern coating sequence varies from user to user, but the following procedure is not atypical. The pattern is dipped into the prime coat slurry, making sure that all surfaces of the pattern are evenly coated. Excess slurry is drained back into the slurry tank.
The wet pattern is then stuccoed with refractory grain. The particle size of the stuccoing grain is generally 50-120 mesh for the prime, or inner, coats and 20-100 mesh for the back-up coats. After stuccoing, the coated pattern is allowed to dry.
Subsequent coats of the slurry are applied by dipping the pattern in the slurry, stuccoing the pattern, and drying the pattern. Four to nine or more coats of the slurry are generally required, depending on the casting size and the metal pressures to be encountered. The last coat is normally not stuccoed because an outside coating of stucco may become dislodged from the pattern, which is undesirable. After the final coat, the shell is dried until the final moisture content is about 2% or less.
After the shell is dried, the wax pattern is removed. This is usually accomplished by rapid heating in an autoclave, so that the outer surface of the wax is liquified and drained first. If the pattern is not heated in this manner, the bulk of the wax expands and cracks the shell.
The shell is fired to burn out the remaining pattern residue, and then molten metal is poured in the shell. When the metal has solidified, the shell is broken and removed from the casting.
As discussed above, additives such as wetting agents or latex are often added to a prime coat slurry to promote adhesion of the slurry to the pattern. However, these additives have the significant disadvantage of shortening the life of a slurry by causing the slurry to gel, or, in the case of latex, causing the latex to separate from the slurry.
It was attempted in the prior art to add a water-soluble polymer, such as poly(vinyl alcohol) (PVA), to colloidal silica slurries to promote the adhesion of the slurry to a wax pattern and to improve the film strength of the slurry. However, as described in U.S. Pat. No. 3,738,957, compositions of colloidal silica and PVA are unstable and separate over time, which is undesirable because such compositions may be stored for long periods of time in drums, or may be used to make slurries which are desired to last several months.
A conventional method for making adhesives is to mix poly(vinyl alcohol) in an aqueous slurry. Optionally, tackifiers such as boric acid and fumaric acid may be added to the adhesive to increase the tack of the adhesive.
Adhesives have many uses, one of which is in the manufacture of paper tubes. One way of making paper tubes is to provide a solid rod, or mandrel, that has a diameter equivalent to the desired inside diameter of the tube. A first wrap of paper having a desired width is wound spirally around the mandrel. A second wrap of paper is coated on both sides with the adhesive and is wound spirally around the mandrel on top of the first wrap. Additional wraps of paper may be wrapped around the mandrel until the tube has a desired thickness. A final wrap, which has no adhesive, is wrapped on top of the last adhesive-coated wrap.
The quality of the adhesive used in making the tube can be determined by measuring the crush strength of the tube. Crush strength is the amount of force required to cause failure of the tube in a direction perpendicular to the axial direction.